Understanding of living cells is beneficial for a variety of research and development. In order to understand the complex machinery at work within living cells, the position of individual biomolecules is measured. For example, single-particle tracking (SPT), in which the trajectory of a moving individual molecular label, quantum dot, or nanoparticle is determined from a series of images, provides a valuable tool for a wide range of biological applications. Information inferred from the extracted particle trajectory sheds light on physical properties such as particle size, conformation, and the local environment, because observing the motion of single particles directly unmasks nanoscale behavior such as diffusion, directed motion, binding, or anisotropy. If only two-dimensional trajectories are recorded, information is missed.
These and other matters have presented challenges to three-dimensional imaging of particles, for a variety of applications.